Recently, there are mounting concerns for environmental pollution caused by non-degradable petroleum-based plastics and for the depletion of petroleum resources, where there is an interest in applications of renewable natural resources, such as starch, pectin and protein, to food packages that require biodegradability and water solubility. Due to their inherent degradable properties, biodegradable polymer materials have been in the spotlight in various fields, such as medicine, agriculture, environmental, and have been especially valuable in the environmental and medical fields. These polymers can be roughly divided into two groups: natural and synthetic biodegradable polymers. Among these, natural biodegradable polymers are recognized as promising materials, since their raw materials are natural products and are thus environmentally-friendly while having excellent physical performance or adaptability to living organisms, but are problematic in that they are expensive and difficult to arbitrarily control in view of the characteristics of natural products. On the other hand, because of their ability to artificially control and make up for what natural biodegradable polymers lack, synthetic biodegradable polymers have recently been highly rated for their commercial value.
Among the synthetic biodegradable polymer materials, particularly polylactides (PLA) have been used for various applications in the environmental and medical fields due to their excellent performance, environmental friendliness, biocompatibility and non-toxicity. In particular, they have been attracting attention for use in the environmental field, such as disposable packaging films, agricultural and industrial films, and food packaging containers, and have already been used in the medical field, such as drug delivery systems (DDS) for drug release control, pins for securing bone and tissue, screws and sutures. Further, research utilizing such biodegradable polymers in automobile part materials, industrial materials, etc. by enhancing their thermal and mechanical stabilities is also in progress.
In the meantime, the trend in developing these new materials is not only aimed at achieving high functionality of products but research is being carried out towards developing environmentally-friendly products. Thus, there has been an increasing demand by the industry for new materials that can meet the above various conditions. For example, polymer stereocomplexes that are obtained when two enantiomeric types of homopolymers are melted or uniformly mixed with the addition of organic solvents at a temperature above a specific temperature form new crystalline structures and have excellent properties such as higher thermal and mechanical stabilities, as compared to homopolymers, and thus, can be considered a new material that satisfies the demands of the high-tech industry Okada et al., Macromolecules, 20, 904 (1987)). In particular, products utilizing stereocomplexes have improved physical properties and performances and can be used for prolonged periods of time, thereby making it possible to reduce the amount of waste and prevent environmental pollution. Such polymer stereocomplexes may be applicable to various fields, including not only the automobile, packaging and semiconductor industries but also food, medicine, communication and military, depending on the type and molecular weight of the polymers.
In preparing such stereocomplexes, organic solvents may typically be used, or in case where no organic solvent is used, methods using direct melt blending or bulk polymerization may be used. Among these, solvent casting is the most commonly used method, but is problematic in that it is difficult to find suitable organic solvents capable of dissolving biopolymers for preparing the biodegradable polymer stereocomplexes and it takes a long time to thoroughly remove the residual organic solvent after the stereocomplexes are produced (Tsuji et al., Macromol. Biosci., 5, 569 (2005)). In the case of melt blending, its use is limited since it requires a high temperature process of 200° C. or higher which may promote the degradation of biodegradable polymers, in which case there is a high likelihood that the crystallization of homopolymers will be induced rather than the formation of stereocomplexes (Tsuji et al., Macromolecules, 25, 4114 (1992)).
Further, it has been reported that there are limitations in the preparation of high molecular weight biodegradable polymer stereocomplexes having high strength, since the weight average molecular weights of the biodegradable polymers that can be prepared by the above methods range in the hundreds of thousands on average (Fukushima et al., Macromol. Symp., 224, 133 (2005)). Thus, it can be found that there are limitations in preparing high strength biodegradable polymer stereocomplexes having thermal and mechanical stabilities using the above methods. Accordingly, a great deal of research is being carried out regarding novel methods for preparing high weight average molecular weight biodegradable polymer stereocomplexes having high strength.
Meanwhile, carbon dioxide is a widely used supercritical fluid, owing to its low critical temperature and pressure, low cost, incombustibility and non-toxicity. However, supercritical carbon dioxide (sc-CO2) is problematic in that it cannot dissolve polymers other than fluoro-based polymers and silicone-based polymers (siloxane polymers). Accordingly, the present inventors have endeavored to overcome the problems associated with the conventional methods of preparing biodegradable polymer stereocomplexes utilizing the melting process or organic solvents only, and arrived at the present invention by developing methods for preparing biodegradable polymer stereocomplexes having a uniform form, such as powder or porous foam, and enhanced thermal and mechanical stabilities, where different types of enantiomers are mixed within a short time by using a mixed system of supercritical fluid-organic solvent while applying a specific temperature and pressure to form a new crystalline structure.